1. Field of the Invention
This invention relates to a method for the manufacture of a physical quantity detector for measuring fluid pressure, a load and the like.
2. Description of the Prior Art
A pressure sensor, for example, which is a typical example of a physical quantity detector is equipped with a sensor member having a cylindrical part of which one end is closed with a strain generating part and adapted to use the strain generating part as a diaphragm.
This pressure sensor is manufactured by the following steps, as disclosed in JP 2004-45048A, for example.
First, a sensor member 1 is manufactured by machining or forging a metallic material, as shown in FIG. 1, and the surface 3a of a strain generating part (diaphragm) 3 of a cylindrical part 2 thereof is planished by polishing. An insulating film 5 of SiO2 etc. is formed on the polished surface 3a of the strain generating part (diaphragm) 3 by a CVD process or a sputtering process. Thereafter, a thin film of a metal or semi-conductor is formed on the insulating film 5 by a CVD process or a sputtering process and the formed film is etched to a predetermined pattern by means of a photolithographic technique to form a strain gauge 6 formed from the thin film of the metal or semi-conductor. Then, the electrodes 7 of gold, aluminum, etc. for connecting circuits thereto are formed on the strain gauge 6. Further, to finish it as a physical quantity detector, such as a pressure sensor and a load sensor, a protective coat 8, such as SiN, for protecting the strain gauge 6 from steam etc. is formed thereon to form a sensor part 4.
The pressure sensor manufactured in this manner is installed in a desired pressure detecting site by fixedly securing the cylindrical part 2 of the sensor member 1 to a pipe or the like. The fluid, such as gas and liquid, flowing over the pressure detecting site is introduced into the sensor member 1 through a bore 2a of the cylindrical part 2 serving as an introducing hole and reaches to the back surface of the strain generating part (diaphragm) 3. When the strain generating part (diaphragm) 3 is elastically deformed due to the fluid pressure, its deformation will be transmitted to the strain gauge 6 through the medium of the insulating film 5 and the resistance of the strain gauge 6 will vary depending on its deformation. Thereby, the strain gauge 6 transforms the change in pressure to the change in resistance and outputs this change as an electric signal. The output of the strain gauge 6 is taken out of the pressure sensor through a bonding wire, a relay board, an input/output terminals, etc. (not shown) and sent to a predetermined control unit as the information on the pressure of the fluid.
Further, it is known in the art to constitute the sensor part 4 mentioned above by two layers of thin conductor films. For instance, as disclosed in JP 2004-45048A mentioned above, a first thin conductor film is formed on the insulating film 5 formed on the surface of the strain generating part (diaphragm) 3 of the sensor member 1 and a second thin conductor film is further formed over the first thin conductor film at a predetermined height so as to oppose to the first thin conductor film. The second thin conductor film is formed on the inner surface of a cap which is fixedly secured to the insulating film and an electrode electrically connected to the second thin conductor film is formed on the outer surface of the cap. According to this pressure sensor, the first and second thin conductor films form a capacitor. When the diaphragm deforms elastically due to the pressure of the fluid flowed into the sensor member, the insulating film formed on the diaphragm and the first thin conductor film formed thereon elastically deforms accordingly. Owing to the deformation of the first thin conductor film, the distance between the first and second thin conductor films changes and the electrostatic capacity increases or decreases accordingly. As a result, the change in pressure or load depending on the change in electrostatic capacity is outputted.
Heretofore, the pressure sensor is adapted to cope with various pressure ranges by changing the thickness of the diaphragm with the same geometry of the sensor member. Precipitation hardening stainless steel SUS 630 is preponderantly used as a material of this sensor member and the manufacture thereof is performed by the machining of the material mentioned above. However, as the thickness of the diaphragm becomes thin, the maintaining of the machining precision becomes difficult. Further, since the machining and lapping are required to finish the sensor member into a predetermined configuration, there is a problem that a processing cost becomes high.
In order to solve the above-mentioned problems, JP 2004-45048A mentioned above proposes to form the strain generating part by using an alloy of a composition which contains Zr, Ti, or Pd as a main component and is capable of producing metal glass (amorphous alloy) and forging the diaphragm part of the pressure sensor out of the alloy in the super-cooled liquid region thereof.
Since the method described in JP 2004-45048A mentioned above adopts the pressure forging process utilizing the super-cooled liquid region of an amorphous alloy, it gives such merits that the manufacturing steps may be considerably reduced as compared with the conventional manufacturing process by machining, such as cutting and grinding. In such a pressure forging process, however, since a bulk material of alloy is inserted into a concave of a metal mold, heated to a temperature in the super-cooled liquid region, and forged by pressing a molding punch in the concave, it often causes poor transfer of the inner surface. For example, wrinkles or the like occur in the inner surface of the strain generating part (back surface of the cylindrical part on the bore side) of the sensor member. Accordingly, it is required to further improve the surface smoothness of the strain generating part. Further, there is a limit to the thickness of the strain generating part which can be subjected to the pressure forging. The method incurs difficulty in controlling the forming conditions and therefore has room for further improvement in decreasing the frequency of occurrence of pores (cells) within the forged material and short molding. Particularly, the strain generating part which is an important part in the pressure sensor is required to have high dimensional accuracy and surface smoothness and further improved precision of transfer.